This invention relates to free piston Stirling engines, and more particularly to an oil backed diaphragm for suspending a free displacer in a free piston Stirling engine.
This invention is related to application Ser. No. 172,373 for "Diaphragm Displacer Stirling Engine Powered Alternator-Compressor," filed on July 25, 1980, by Folsom, et al., the disclosure which is incorporated herein by reference. The engine of this application Ser. No. 172,373 is a free piston Stirling engine which utilizes a diaphragm to suspend the displacer in the working space and uses the pressure wave in the working space to maintain the displacer oscillation. Although this machine constitutes a significant step forward in the art, there are some areas in which modifications would improve reliability, performance, and power density.
In the machine of application Ser. No. 172,373, the displacer diaphragm is subjected to stress induced by the pressure swing of the working gas in the working space which is on the order of 10 to 20 percent of the charge pressure in the working space, or about 40 bar. Therefore, the pressure swing can be on the order of 4 to 8 bar which, acting over the full face of the diaphragm, can introduce considerable stress in the diaphragm. This complicates the deformation pattern of the diaphragm and reduces its working life. This pressure induced stress does not contribute to the operation of the machine. The only stress that is desirable from the design function is displacement induced stress, that is, the spring effect contributed by the diaphragm when it is displaced from its central position. This necessary and desirable stress in the diaphragm is compounded and multiplied in unpredictable ways and with deleterious results by the pressure induced stresses in the diaphragm so that the diaphragm design is greatly complicated and diaphragm reliability and repeatability is decreased. Moreover, the effect changes with changing pressure and therefore an additional degree of difficulty is introduced when a power control system based on mean pressure variation is used.
A second area of improvement which would be desirable is control of the power input into the displacer itself. Power input into the displacer is related to the ratio .DELTA.V/V where .DELTA.V is the difference in the volumetric displacement of the displacer in the expansion space and the compression space, and V is the volumetric displacement of the displacer in the expansion space. The power needed to maintain the oscillation of the displacer in the working space, that is the power necessary to overcome the friction and windage losses of the working gas in the heat exchangers, normally requires a .DELTA.V/V ratio of approximately 0.1. However, a diaphragm normally provides a .DELTA.V/V in an engine of this variety of approximately 0.3, thus providing more energy for the displacer than it needs. This energy excess causes the displacer to slam back and forth between its stop unless some means is provided to extract the excess energy put into the diaphragm by its thermodynamic system, or some technique is provided for constraining the .DELTA.V/V ratio to a value more suited to the engine operation.
Along these lines, it is possible by artful design of a displacer to enable it to operate with the characteristic desired, that is with a small .DELTA.V. However, although the diaphragm is capable of operating in this manner, there is no assurance that it will indeed operate in this manner when placed in an unconstrained manner in the engine and operated at various pressures and other operating parameters. Therefore, it is necessary to impose some form of restraint on the deformation pattern of the diaphragm in its operation so that it will conform to the desired configuration.